Fibers form, in part or in whole, a large variety of both consumer and industrial materials such as, for example, clothing and other textile materials, medical prostheses, construction materials and reinforcement materials, and barrier, filtration and absorbent materials. There are two main structural classes of fiber materials: woven and non-woven. An advantage of non-woven fiber materials is their lower production cost.
Nanofibers are increasingly being investigated for use in various applications. Nanofibers may attain a high surface area comparable with the finest nanoparticle powders, yet are fairly flexible, and retain one macroscopic dimension which makes them easy to handle, orient and organize. Moreover, the high surface area of nanofibers may facilitate the addition of particles that improve the properties of the nanofibers such as mechanical strength, and/or impart additional functionality such as therapeutic activity, catalytic activity, or microelectronic/optoelectronic functionality.
In the use of nanofibers for applications such as those noted above, high volume and low production cost are generally desirable to achieve commercial viability. Five general methods for the production of fibers with nanometer or single-micron diameters exist: drawing, phase separation, electrospinning, template synthesis and self-assembly. Of these, melt blowing, splitting/dissolving of bicomponent fibers, and electrospinning have shown a potential for commercial-scale fiber production. The first two techniques are based on mechanical drawing of melts and are well-established in high-volume manufacturing. In melt blowing polymers are extruded from dies and stretched to smaller diameters by heated, high velocity air streams. Bicomponent spinning involves extrusion of two immiscible polymers and two-step processing: (1) melt spinning the two polymer melts through a die with a “segmented pie” or “islands-in-the-sea” configuration, followed by solidification and (2) release of small filaments by mechanically breaking the fiber or by dissolving one of the components. A disadvantage of these techniques is that they are limited to melt-processable polymers.
Many polymers of commercial interest, including acrylics and especially polymers that are biocompatible and biodegradable, are only processed from their solution. So far no commercial solution spinning method has been developed for creating nanofibers from such polymers. The two main types of solution spinning, dry-spinning and wet spinning, like melt spinning, also involve extrusion of the polymer through an orifice. In dry-spinning the polymer is then drawn through air at elevated temperature while the solvent evaporates. In wet-spinning the fiber is then drawn in a coagulation bath.
Electrospinning differs from melt or dry spinning by the physical origin of the electrostatic rather than mechanical forces being used to draw the fibers. Among these three techniques, electrospinning can produce the smallest fibers (20-2000 nm in diameter), and to date has been the only technique that can produce sub-micron fibers from most polymers. However, low production rate is a major disadvantage of this technique. For the wide commercialization of nanofibers there is a need for a method capable of several orders of magnitude higher productivity.
It would also be desirable to create nanofibers that are organic/inorganic composites. However, organic and inorganic materials are conventionally made and used separately because of their widely differing precursor chemistries and synthesis procedures. Inorganic materials are typically produced as thin films via vacuum deposition processes, or in some cases as particles via colloidal synthesis. It would be desirable to produce composite nanofibers by way of an integrated process.
Accordingly, an ongoing need remains for improved techniques for fabricating nanofibers. There is also a need for fabricating composite inorganic/organic nanofibers and pure inorganic nanofibers.